The production scale and production technology of the paper industry is constantly increasing and improving as demand for paper increases. Environmental pollution and excess capacity caused by conventional pulp and papermaking processes have attracted increasing attention from both governments and enterprises. Energy saving processes, emissions reduction, and clean production of pulp and paper are of interest.
In Europe and the United States, for instance, heavy use of oil and natural gas fuel occurs during pulp bleaching. These two kinds of fossil fuel generally have a higher combustion efficiency and less energy consumption relative to other fossil fuels, such as coal. In China, coal is currently given priority in pulp bleaching. Compared with natural gas, the combustion efficiency of coal is lower and it causes more pollution. Therefore, the energy consumption of the pulp bleaching industry in China is significantly higher than in Europe and the United States. Accordingly, a document entitled “State Council on the Strengthening of Energy-Saving Work in the Decision” was released in China in 2006. This document recommends of enterprise sustainable development to speed up the construction energy-saving pulp production processes.
Following the abandonment of chlorine as a bleaching agent due to environmental considerations, two technologies were developed. The main conventional pulp bleaching technologies are elemental chlorine free (ECF) bleaching based on chlorine dioxide (ClO2) and total chlorine free (TCF) bleaching with hydrogen peroxide or enzymes. The main distinguishing factor between these two processes is whether chlorine dioxide is used—it is used in ECF, but not in TCF. To compensate for the lack of chlorine dioxide, TCF will either add higher dosages of peroxide or supplement the process with ozone. ECF is more commonly used. In 2012, ECF pulp constituted 93% of the world share of the bleached chemical pulp market. TCF, although decreasing slightly in market share, has remained at approximately 5% of the bleached chemical pulp market.
Initially TCF appeared to hold promise as an alternative to ECF with respect to the generation of environmentally harmful substances. However, several studies have shown no significant difference between the two technologies when best available techniques are used. No measurable levels of dioxins are present with either ECF or TCF. However, TCF exhibits weaker fiber strength, has a lower brightness potential, has lower yield, and has higher energy requirements. Also, TCF significantly influences the viscosity of fully bleached pulp. This has further eroded its promise as a successor to ECF. Chlorine dioxide bleaching is used as the first stage of ECF bleaching, but adsorbable organic halides (AOX) can still be detected in the D0 bleaching stage effluent.
High temperature chlorine dioxide bleaching (DHT) has been a success in pulp bleaching because the content of AOX in the high temperature chlorine dioxide bleaching wastewater is much less than that in the conventional Do stage bleaching wastewater. There is a potential for heat energy self-sufficiency, and further improvements in chlorine dioxide usage and efficiency of ECF bleaching remain a goal. Accordingly, an improved process and apparatus for reducing emissions and clean production of pulp bleaching may be beneficial.